Subterranean wells are used to produce various gases and liquids. For example, a subterranean well can be used to produce methane gas and liquid water from a coal seam. This type of subterranean well can include a well bore from the surface to the coal seam, a well casing cemented to the well bore, and a metal tubular within the well casing. The well can also include a submerged pump located within an under reamed cavity in the coal seam. During production from the well, water is pumped from the cavity, and through the tubular, to water production equipment at the surface. In addition, gas flows from the coal seam into the cavity, and through the annulus between the tubular and the well casing, to gas production equipment at the surface.
The methane gas can cause various problems with the submerged pump during production from the well. For example, the pump can experience vapor lock due to excessive gas flow through the pump. This vapor lock can create inefficient pump operation, and excessive duty time for the pump motor. In addition, motor cycling and gas moving through the pump can cause excessive motor heating, and premature failure of the pump and/or motor. Production of gas through the tubular is also a problem, as this gas is entrained with the water, rather than being produced to the gas production equipment at the surface.
One prior art approach to gas flow through the pump is the use of gas shrouds on the pump, which prevent gas from entering the pump inlet. U.S. Pat. No. 6,361,272 B1 to Bassett entitled “Centrifugal Submersible Pump”, discloses a submersible pump having this type of gas shroud. However, gas shrouds are not always effective in coal bed methane wells, or other pumping installations, which require the pump to be landed within the cavity in the coal seam, or above a producing zone of the well. In addition, gas can be driven downward and into the pump in a u-tubing manner, as heads of water fall back down the annulus, after they can no longer be lifted toward the surface by gas flowing up the annulus.
The liquid water can also cause various problems during production from the well. For example, water and/or wet gas flowing in the annulus of the well can enter the gas production equipment at the surface. This water can cause excess flowline pressures, lines filling with water, and metering errors in the gas production equipment. Water in the annulus, and water heads moving up and down the annulus, can also create harmful fluid column effects, such as unsteady production of water and/or gas from the well, due to the relative position and amount of fluid movement in the annulus.
One prior art approach to water accumulation in the gas production equipment is the use of drips and blowdown lines in low-lying areas of the gas production equipment, such as surface gas lines. These drips must be vented regularly to blow out the accumulated water. Typically, due to the low pressures in coal bed methane gas lines (e.g., less than 20 psig), the blowing of drips is manpower intensive, and inefficient in comparison to lines operating at higher pressures. It would be advantageous to eliminate water entirely from gas production equipment at the surface, and the need to blow drips from this equipment.
The present invention is directed to a novel method and system for producing gas and liquid in a subterranean well, in which gas flow through a submersible pump, and liquid flow through a well annulus to the surface, are substantially eliminated. In addition, the method and system can be adapted to different types of wells, including wells that employ formation pressures rather than pumps, to move the gas and the liquid.